Air Voids in Concrete: A Study of the Influence of Superplasticizers by Means of Scanning Electron Microscopy and Optical Microscopy

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Title: Air Voids in Concrete: A Study of the Influence of Superplasticizers by Means of Scanning Electron Microscopy and Optical Microscopy

Author(s): R. Pleau, M. Pigion, R. M. Faure, and T. Sedran

Publication: Special Publication

Volume: 122

Issue:

Appears on pages(s): 105-124

Keywords: air-entrained concretes; air-entraining agents; air entrainment; capillarity; electron microscopes; freeze-thaw durability; microscopy; plasticizers; porosity; voids; Materials Research

Date: 6/1/1990

Abstract:
Samples from nine air-entrained concrete mixes made with and without a superplasticizer were examined under a scanning electron microscope to determine the size distribution of the voids in the 0.5 to 50 æm range. Concurrently, samples of the same mixes were examined under a binocular microscope to determine the size distribution of the voids in the 10 to 1000 æm range. The voids observed under the electron microscope were separated into two categories: air voids (spherical in shape or nearly so) and large capillary pores (irregularly shaped). The results show that, in mixes, the amount of capillary pores with diameters ranging from 0.5 to 50 æm is relatively important (the number of these voids generally represents approximately half the total number of entrained air voids). The role of these pores in the frost resistance of concrete is believed to be strongly dependent on their degree of saturation at the time of freezing. The number of air voids smaller than 10 æm in diameter, however, was found to represent less than 10 percent of the total number of entrained air voids. These small air voids are thus expected to have little influence on frost durability. The results also indicate that the distribution of the ir-void diameters is influenced by the nature of the air-entraining agent but not by the use of a superplasticizer. The distribution of air-void diameters was found to be approximately the same for all mixes, irrespective of the value of the spacing factor.